1. Field of the Invention
The present invention relates to a method of producing single crystals by the Czochralski method (hereinafter referred to as “CZ method”) and, more particularly, to a method of producing silicon single crystals which serve as raw materials for silicon wafers to be used as substrates for semiconductor devices.
2. Description of the Related Art
The CZ method may be mentioned as a typical one among others known for the production of silicon single crystals. In the production of a silicon single crystal by the CZ method, a seed crystal is immersed into a silicon melt in the vicinity of the surface thereof in a quartz crucible and, while the crucible and the seed crystal are rotated, the seed crystal is then slowly pulled up to thereby allow a silicon single crystal to grow adhering to the seed crystal.
It is known that, in such silicon single crystal pulling by the CZ method, the defect distribution etc. in crystal sections are greatly influenced by the rate of crystal growth on the occasion of growing single crystals, namely by the pulling speed and the crystal interior temperature gradient in the direction of the pulling axis just after solidification; and it is considered that dislocation-free single crystals having only a small number of defects can be produced by controlling the value of V/G (wherein V is the pulling speed and G is the crystal interior temperature gradient) at a specific constant level. Since it is generally difficult to control the crystal interior temperature gradient G in the direction of the pulling axis, it is a general practice to control the pulling speed V.
FIG. 1 illustrates the state of distribution of typical defects (called “grown-in” defects) occurring in a silicon single crystal obtained by the CZ method. This figure schematically shows an observation result by X ray topography for elucidating the distribution of micro-defects, whereas a wafer consisting a plane perpendicular to the pulling axis is cut out from a single crystal just after growing, immersed in an aqueous solution of copper nitrate for deposition of Cu and then subjected to heat treatment. Grown-in defects are micro-defects the cause of which is incorporated in the step of single crystal growing, being discernible in a device manufacturing process, and greatly influential on the performance of the device.
FIG. 2 is a view illustrating a general relationship between the pulling speed in the step of single crystal pulling and the site of occurrence of crystal defects and schematically shows the state of distribution of defects in a longitudinal section of a single crystal grown with the pulling speed being gradually reduced. For example, the distribution of defects as shown in FIG. 2 can be obtained by growing a single crystal while gradually reducing the pulling speed, cutting the crystal along the pulling axis in the central part of the crystal and examining the distribution of defects in the thus-obtained section by the same technique as mentioned above referring to FIG. 1.
The wafer shown in FIG. 1 is the one cut out from a single crystal at the level indicated by A in FIG. 2, or from a single crystal grown at a pulling speed corresponding to the level of A.
In the wafer shown in FIG. 1, there are found oxygen-induced stacking faults (hereinafter referred to as “OSFs”) distributed in a ring-like manner at a position of about two thirds (⅔) of the outside diameter. Inside the ring-like OSFs, there are found crystal-originated particles (hereinafter referred to as “COPs”) and, outside the ring of OSFs, there exists a region in which dislocation cluster defects occur. COPs are defects resulting from aggregation of vacancies introduced into crystal lattices in the vicinity of the solid-liquid interface in the step of single crystal growing; similarly, dislocation cluster defects are ones resulting from aggregation of excess silicon atoms (interstitial silicon atoms) incorporated into crystal lattices. OSFs are interstitial-silicon-atom-induced stacking faults caused during oxidizing heat treatment.
Adjacent to and outside the region of occurrence of ring-like OSFs, there is an oxygen precipitation promotion region in which oxygen precipitation has occurred actively and, between this region and the outermost region as having dislocation cluster defects, there is an oxygen precipitation inhibition region where oxygen precipitation scarcely has occurred. Both the oxygen precipitation promotion region and the oxygen precipitation inhibition one are defect-free regions where scarcely showing such grown-in defects as COPs and dislocation clusters.
As the pulling speed is increased, the region of occurrence of ring-like OSFs shifts toward the periphery and is finally driven out of a usable region of the crystal, as shown in FIG. 1 and FIG. 2. Conversely, when the pulling speed is reduced, the region of occurrence of ring-like OSFs shifts toward the central part of the crystal and finally diminishes there.
Any kind of crystal defects mentioned above deteriorates device characteristics and/or causes defectives thereof. However, COPs do not affect adversely so much, as compared with dislocation clusters, and they are rather effective in increasing the productivity. Therefore, the single crystal growing has been conventionally performed in a manner such that the pulling speed is increased so that the region of occurrence of ring-like OSFs may be located in the peripheral portion of the crystal.
However, with the advancement of device miniaturization following the recent trends of semiconductor devices toward reduction in size and integration to higher level, COPs are now significantly causative of reduction in yield of good-quality products; therefore, it is now an important task to reduce the density thereof in occurrence. Accordingly, the single crystal growing is conventionally performed in a manner such that the structure of a hot zone in the growing apparatus is improved, for example, by surrounding the pulled-up single crystal with a heat-shielding member, to thereby enlarge the above-mentioned defect-free region so that the defect-free region may occupy the whole wafer surface.
FIG. 3 schematically shows the state of defect distribution in a section along the pulling axis for a single crystal pulled up by means of a growing apparatus having such an improved hot zone structure. Like in the case illustrated in FIG. 2, the distribution patterns of respective defects in the single crystal change as shown in FIG. 3 when the single crystal growing is carried out while varying the pulling speed. Thus, when the pulling growth is carried out using a growing apparatus improved in hot zone structure within the speed range from B to C as shown in FIG. 3, single crystals having a main body portion (i.e. product portion) which is mostly occupied by the defect-free region can be produced; accordingly, wafers with least grown-in defects can be manufactured.
As for the method of producing single crystals having such a defect distribution as shown in FIG. 3, Japanese Patent Application Publication No. 2005-15290, for instance, proposes a method of controlling the crystal interior temperature gradient C, wherein the distance between a raw material melt surface and a heat-shielding member is adjusted, the heat-shielding member being disposed facing to the raw material melt surface. According to this proposed method, the ratio V/G is controlled substantially at a constant level by varying the distance between the raw material melt surface and the heat-shielding member during the progress of single crystal growth, being independent to the pulling speed V, so as to grow single crystals having a desired defect region(s), for example those having an N region in which no defects exist. It is alleged that, according to this method, single crystals can be grown efficiently without reducing the pulling speed V, and further, variations in diameter for single crystals can also be reduced, and accordingly, the productivity and yield in single crystal production can be improved.
However, the hot zone structure (heater, insulating material, etc.) in a pulling apparatus to be used in carrying out the CZ method is made of a graphitic material, and therefore, with the lapse of time, the graphitic material undergoes conversion to silicon carbide as a result of reaction thereof with evaporative substance such as those evaporated from SiO to incur expansion phenomena, and further, the evaporated silicon oxides deposit on the surface of the material. Accordingly, the state of the hot zone structure is changed, for example the radiation factor thereof is lowered; as a result, the crystal interior temperature gradient in the direction of the pulling axis is altered. Therefore, in the case of producing crystals having a desired defect distribution, in particular defect-free crystals in which neither COPs nor dislocation clusters exist, it is necessary to properly adjust the production conditions such as the distance between the raw material melt surface and the heat-shielding member and the pulling speed in response to such changes in state.
Further, for example, in the case of interruption of the pull-up for some or other reason such as the occurrence of dislocations during the pull-up, followed by remelting (melting back) of the single crystal in the middle of growth, the evaporation of silicon oxides from the silicon melt continues over a long period when complete remelting of the grown single crystal is carried out. In this case, the change in chamber inner surface radiation factor notably occurs.
Further, in the case where the start of single crystal pull-up is delayed by remelting, for instance, the quartz crucible progressively gets softened to result its distortion into a fat-and-short shape, whereby the thickness of the quartz crucible increases and the inside diameter thereof decreases, resulting in the height change of the raw material melt, which is another problem.
Therefore, for stably producing crystals having a desired defect distribution, in particular defect-free crystals, it is necessary to adjust the production conditions prior to the start of single crystal pull-up in response to such changes in chamber inside conditions, including temporal changes within the hot zone structure.